Slipping clutch does generate heat but i'd imagine that the clutch pads would be worn away and plastic parts on the bike would melt off before a clutch generated so much heat the piston siezed. That being said, fix the slipping clutch.
I use only one ring too, and the bike runs just fine. Race engines sometimes use one ring to reduce friction..
But this could be at least part of the overheating problem. Pistons crowns get hot as hell and there's no direct contact with the walls of the cylinder to disapate heat. Cool fuel entering the crankcase cools the underside of the piston and fresh fuel in the combustion chamber absorbs heat from up there. But the rings are also a major heat carrier, transfering it directly to the cylinder walls.
If this problem started just after you lost a ring and started running on only one, that may be it..
Otherwise check your timing. It may be way off.. and 2-strokes that have been modified for increased performance generally need less ignition advance at top speed than at low speed since engine breathing is more efficient at high RPMs.